System and method for operating a subsea sensor field

ABSTRACT

A system ( 1 ) for operating a subsea sensor field ( 2 ), comprises an automated underwater vehicle-AUV ( 10 ) and a subsea service station ( 13 ). A sensor ( 11, 12 ) in the sensor field ( 2 ) comprises a permanently installed base unit ( 11 ) and a removable control unit ( 12 ). The AUV ( 10 ) moves control units ( 12 ) to the service station ( 13 ) for charging and updating, and then back to the base units. The permanent positions of the base units ( 11 ) facilitate time-lapse surveys, and saves energy as heavy equipment may remain on the seafloor.

BACKGROUND Field of the invention

The present invention concerns a system and method for operating asubsea sensor field, i.e. an array of sensors and sources for monitoringa region of interest.

Prior and Related Art

Subsea reservoirs or geological structures may be monitored usingdifferent time-lapse techniques, each having its advantages anddisadvantages. A first type of technique involves repeated marineseismic surveys with various types of surface-towed seismic equipment,i.e. powerful acoustic sources and associated seismic streamers. Themain advantage of these techniques is an ability to monitor or surveylarge areas relatively fast. A disadvantage is that the collected datainclude P-waves (primary pressure waves) only, as S-waves (secondaryshear waves) from a subterranean structure do not propagate through thewater column to the hydrophones in the streamers.

A second technique involves repeated marine seismic surveys withre-deployable ocean bottom sensors (wired or autonomous) with towedseismic sources. In these techniques, geophones on the seafloor collectadditional data from the S-waves. In some varieties, ocean bottom cablesare retrieved at one edge of a large array and redeployed at an oppositeedge while a survey is conducted in the middle of the array usingseismic sources. Such methods require one or more cable handlingvessels, a source vessel and a large number of sensors. Alternatively,one vessel can move the entire seafloor array between surveys, therebyreducing speed but also expenses.

A third technique involves repeated marine seismic surveys withpermanently installed ocean bottom sensors (typically wired) with towedseismic sources. Such methods are suitable for monitoring a fixedstructure, e.g. a reservoir or a structure around an installation, asevery sensor is located at the same spot in subsequent surveys. Theseismic sources may still be towed by a source vessel. However, suchvessels typically must be hired a long time in advance, and their costof operation is relatively high. Both factors tend to increase theperiods between surveys. Other disadvantages are problems associatedwith towing and releasing powerful acoustic sources near aninstallation, adverse effects on marine life, etc. Seismic sources onthe seafloor have been proposed to reduce these problems.

Other techniques known in the art use electromagnetic radiation (EM)instead of acoustic signals, and otherwise resemble the acoustictechniques discussed above. Gravimetric techniques measure localvariations in gravitation to determine changes in the underground. Stillother techniques use accurate inclinometers to measure changes in theinclination at different points above a geological structure in order torecord changes. Any suitable sensor and/or source can be used with thepresent invention. For example, some of the devices above are based onaccelerometers, in particular MEMS-accelerometers. Any other instrumentsor devices, e.g. seismometers, based on accelerometers can be used withthe invention. Furthermore, non-seismic sources and sensors, e.g.echo-sounders, as well as sensors for monitoring environmentalparameters of any kind are anticipated.

In this description and the claims, the term ‘sensor field’ denotes acollection of any kind of sensor and/or source as briefly describedabove. The sensor field is deployed at a seafloor, and monitors a regionof interest, i.e. a geological structure below the sensor field and/or abody of water above the sensor field as well as the seafloor on which itis deployed. Regardless of type, the sensors and/or sources in thesensor field may be connected in a network for communication and/orpower supply. Alternatively, the sensors and sources can be connecteddirectly to the surface, or be contained in autonomous units with anassociated battery unit, a storage unit etc. as known in the art. Theterm ‘wired network’ as used herein, means a conventional network ofmetallic wires and/or fibres for power and/or communication. Thus, asubsea communications network based on, for example, acoustic links asknown in the art is a network, but not a wired network.

In general, a system for operating such a subsea sensor field requiresmeans for deploying and retrieving components, providing power,downloading instructions, uploading data, calibrating sensors etc. Asindicated, the actual functions of the operating system depend on theapplication at hand. For example, electric power could be suppliedthrough a wired network or from a battery unit, the system could dependon a source towed behind a surface vessel, etc.

In the following description and claims, a distinction is made between‘autonomous’ and ‘automated’. An ‘autonomous device’ contains its ownpower supply and control system. Hence, an autonomous device does notrequire power supply or communication lines to the surface, whereasexternal power and communication lines are optional for an ‘automateddevice’. In particular, the term AUV as used herein means ‘automatedunderwater vehicle’, and include conventional remotely operated vehicles(ROVs) and movable subsea platforms in addition to autonomous underwatervehicles.

The term ‘control system’ as used herein includes any system comprisinga sensor, a controller and an actuator wherein the controller issuescommands to the actuator depending on input from the sensor and acontrol algorithm. That is, ‘control system’ as used herein includes,but is not limited to, a system implementing a cybernetic loop. Forexample, a system issuing a storage command to a storage device in anautonomous sensor node is a control system according to the presentdefinition.

As known in the art, deploying components on the seafloor with an ROV istime consuming and/or inaccurate. This is a disadvantage in time-lapsemonitoring, where the sensors should preferably be located at the samespot before and after retrieval and redeployment.

Control algorithms for storage and retrieval at predefined positions aregenerally known, and implemented in, for example, robotic systemsstoring and retrieving goods in a warehouse. These control algorithmscan obviously be adapted to deploy and retrieve nodes at predefinedpositions on the seafloor, and are not further described herein.However, there are no rails or other guides in a sensor field at theseafloor, so an autonomous underwater vehicle would either need apost-installed guidance system, or depend on inertial navigation, inparticular output from MEMS-accelerometers as known in the art.

The objective of the present invention is to provide system foroperating a sensor field solving at least one of the problems from priorart while retaining the benefits.

SUMMARY OF THE INVENTION

This is achieved by a system according to claim 1 and a method accordingto claim 7.

In a first aspect, the invention concerns a system for operating asubsea sensor field, comprising an AUV and a subsea service station. Atleast one sensor in the sensor field comprises a permanently installedbase unit and a removable control unit. The AUV and control unit areprovided with complementary transport connectors and the service stationis provided with a docking connector for receiving the control unit.

The control unit may contain its own power supply, i.e. a battery unit,storage for data etc., and be self-contained for a period of time untilthe AUV brings it to the service station for charging and updating. Theupdating may include uploading sensor data, calibration, synchronizationof internal clocks, downloading instructions, software or firmwareupdates etc., and is performed at regular intervals or when required.Thus, the control unit does not require power lines or communicationlines. As the base unit remains at the seafloor, the control unitreturns to the exact same spot as before, so no accuracy is lost e.g. ina 4D-operation. In some applications, the base unit contains heavyelements, e.g. a mass for generating low frequency signals for a seismicsurvey. In these applications, there is limited need for inflatablebuoyancy elements and no need for a powerful AUV as only the controlunit moves to and from the service station. This reduces the complexityof operation, the requirements for the AUV and saves energy.

The service station may be connected to the surface through anumbilical. In line with common usage, the umbilical is assumed to be ashielded cable containing power lines, communication lines etc. suchthat the operations at a subsea service station may be controlled fromthe surface. Thus, the alternative to an umbilical would be retrievingthe control unit to a service station at the surface. The most costeffective alternative depends on the application.

The system preferably comprises a docking station for the AUV. This maybe deployed at the seafloor to eliminate or reduce the need forretrieving the AUV to the surface as would be the case for a typicalROV. Alternatively, the docking station can, for example, be part of acradle for an ROV at an operation deck of a surface vessel. Either way,the docking station is of a standard type used for recharging andtesting the AUV at the surface or at the seafloor. In general, thedocking station for the AUV may be associated with the service stationfor control units, or the docking station and service station may bedifferent devices at different locations.

In some embodiments, the base unit is connected to a wired network. Thewired network may be part of a permanent array on the seafloor, e.g. formonitoring a reservoir, and provide power and/or communication to someor all components on the seafloor. For example, a seismic source with aremovable control unit may be provided with power and communication froma permanent network originally installed for providing power andcommunication to an array of permanent sensors.

The wired network may be connected to the service station. Thus, onecentral installation, preferably on the seafloor, comprise allconnectors, subsea controllers etc. for the control units and the AUV.

In a preferred embodiment, the AUV comprises a control system and a setof base units comprise a corresponding guidance system, such that theAUV can operate without communication to the surface except through thedocking station. The set of base units provide a set of fixed locationssuitable for a position system, e.g. one based on acoustic waves orradio waves. In many instances, a commercially available acousticpositioning system would be suitable. However, any alternative known inthe art may be considered, for example a visual beacon, an acousticbeacon or a guidance system based on a Hall-sensor tracking a conductingwire. Either way, navigation and localisation of a base unit is fasterand more accurate using such a guidance system than by using inertialnavigation means only. The AUV can obviously contain a battery unit, andinstructions, including software and firmware updates, can be providedthrough the docking station. Thus, the AUV can be operated withoutseparate power lines and communication lines, and without a dedicatedoperator steering the AUV by remote control. In general, a guidancesystem facilitates autonomous operation of the AUV, thereby eliminatingthe need for a communication link from the AUV to the surface andreducing the need for a dedicated human operator at the surface.

In a second aspect, the invention concerns a method for operating asubsea sensor field, comprising the steps of: deploying a system asdescribed above; connecting a control unit to a base unit by means ofthe AUV; operating the control unit at the base unit for a predeterminedperiod of time and moving the control unit to the service station bymeans of the AUV.

The step of moving the control unit to the service station by means ofthe AUV may be performed at regular intervals. This way, the AUV maycollect several control units and bring them to or from the servicestation in one trip. Alternatively, the AUV may collect one or morecontrol units one by one when an event occurs, e.g. when a first batteryin a set of similar batteries is depleted, or for synchronising a set ofcontrol units when it is decided to perform a series of measurements.The step of operating the control unit includes collecting data if thecombined control/base unit comprises a sensor, and timing etc. if thecombined control/base unit comprises a source.

In some embodiments, one control unit may be connected to several baseunits sequentially. This may be useful if the control unit compriseparticularly expensive and/or accurate instruments. For example, anaccurate and expensive gravimeter brought from base unit to base unitmay provide better data than several less accurate and less expensivepermanent gravimeters. It is also appreciated that interchanging similarcontrol units may be useful during troubleshooting, or in order toaverage data from one location by measuring data at the location of thebase station with a series of different control units.

The step of moving the control unit may include collecting severalcontrol units before moving to the service station. This corresponds tothe embodiments of the system in which the control units are chargedand/or updated on a regular basis and/or when a particular event occurs.

Further features or benefits will appear from the dependent claimsand/or the detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail by means of examplewith reference to the accompanying drawings, in which:

FIG. 1 illustrates a system according to the invention for operating asensor field.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 illustrates a sensor field 2 deployed on a seafloor, i.e. under abody of water. The sensor field 2 has permanent sources 20 and sensors22 of various kinds as described in the introduction. The permanentsources 20 and sensors 22 are connected to a wired network 15, whichprovides electric power and/or communication links. The sensor field 2could be designed differently than shown in FIG. 1, for example as arectangular grid of synthetic ropes with an autonomous node at eachintersection. The grid could be made of synthetic rope or steel wire formaintaining predetermined distances. Thus, a grid does not imply powersupply or communication, but power and/or communication can be providedthrough a grid.

The sensor field 2 is operated by a system 1 comprising an AUV 10, baseunits 11, 11 a, 11 b, control units 12, a service station 13 for thecontrol units 12 and a docking station 14 for the AUV 10.

When connected, a base unit 11 and a control unit 12 correspond to afully operable generic source unit 20 or sensor unit 22 as describedabove. That is, one combination of base unit 11 and control unit 12 canbe an acoustic source, and another combination can be a gravimetricsensor device.

The base units 11, 11 a, 11 b remain on the seafloor while the AUV 10carries their respective control units 12 to the service station 13 forcharging and updating and back to the base units several times. Here,the term ‘updating’ includes any operation involving communication tothe surface, e.g. uploading any measurement data, calibration,synchronisation of internal clocks, downloading instructions includingany software and firmware updates, etc.

As a first example, assume that the base unit 11 and its associatedcontrol unit 12 represent an acoustic source. In this example, the baseunit 11 contains heavy parts, e.g. a large mass used to provide lowfrequency acoustic signals and a motor to move the mass. Prior to aseries of measurements, the associated control unit 12 is brought to theservice station 13, where its batteries are charged and an internalclock is synchronized to sensor clocks in the sensor field 2. Then, theAUV 10 carries the control unit 12 back to the base unit 11, where thecontrol unit 12 is reconnected to its base unit 11 to form a completeacoustic source. In this example, the power consumption is reduced asthe large mass is not moved by the AUV 10. Further, the accuracy ofmeasurement is increased as the base unit 11 remains in place duringcharging and updating of the control unit 12. Last, but not least, theability to synchronise an internal clock shortly before activating thesource for a series of measurements enables the use of an inexpensivetimer in a fully autonomous source without sacrificing accuracy.

In a second example, base unit 11 a is adapted to receive a sensitiveand accurate gravimetric instrument within a control unit 12. Theassociated control unit 12 is temporarily removed, e.g. for charging,updating or protection from powerful acoustic pulses from a seismicsource. As the base unit 11 a is functionally different from the baseunit 11 in the previous example, the connectors to the respectivecontrol units 12 are functionally different.

For example, there is a need for a high power connection to the motor inthe previous example, but no need for a similar power connection in agravimetric instrument. However, transport connectors for connecting thecontrol units 12 to the AUV 10 during transport to and from the servicestation 13 are similar or identical.

Base unit 11 b is connected to a wired network 15 providing power and/orcommunication to the sensor field 2. Thus, power and/or communication toa combination of a base unit 11 b and a control unit 12 may be providedthrough the network 15. For example, a firing signal for a sourceassociated with base unit 11 b may coincide with firing signals to thepermanent sources 20. Alternatively or additionally, the base unit 11 bmay comprise a power source charged from the network 15 such that theassociated control unit 12 only uses the service station 13 forupdating.

The service station 13 accommodate control units 12 of different shapes,sizes and configurations, as the control units 12 belong to differentsources and sensors. More precisely, the service station 13 comprisesdifferent bays, each with physical dimensions and connectors adapted toone or more control units 12. A small control unit 12 can be received ina bay accepting larger control units 12, provided the physical andelectrical properties of the connectors are compatible.

The docking station 14 is adapted to the AUV 10, and severalcombinations of an AUV 10 and an associated (subsea) docking station 14are commercially available. The docking station 14 essentially providethe same services for the AUV 10 as the service station 13 provides forthe control units 12, in particular charging, updating and providingoperating instructions. The service station 13 and the docking station14 are independent of each other, and may be deployed at differentlocations subsea or at the surface.

An umbilical 3 connected to the service station 13 and/or the dockingstation 14 provides power and communication lines to the surface. Theumbilical is of a type known in the art, and is not further describedherein.

Preferably, the umbilical 3 connected to the service station 13comprises power lines and communication lines already provided for thesensor field 2 and/or the network 15. In some instances, the servicestation 13 may thus be connected to one or more auxiliary outputs in acentral unit already provided for the permanent sensor field 2. Thedocking station 14 for AUV 10 can be connected in a similar manner, orbe provided with separate power supply and communication lines from thesurface.

During operation of the sensor field 2, the AUV 10 can bring a controlunit 12 to the service station 13 when the control unit 12 needscharging, prior to a survey etc., i.e. based on an event. Preferably,such an event-based handling is combined with a regular maintenanceschedule. For example, several control units 12 can have similarbatteries so that when one battery is depleted, other batteries arelikely to be depleted shortly thereafter. In this case, it might beefficient to collect and charge several batteries in one trip with theAUV 10. It is also possible to schedule certain tasks at regularintervals, for example synchronising all clocks in a set of controlunits once a week. In general, the scheduled and event based modes ofoperation are independent of each other, so one operational mode doesnot exclude the other.

The AUV 10 can be a traditional ROV with a light source, a camera and atether comprising power lines and communication lines to the surface. Interms of the generic control system briefly discussed in theintroduction, the camera is a sensor providing visual images, thecontroller is a human operator, and the actuator is one or morethrusters on the ROV. In this system, a base unit with a visual beacon,e.g. a solid state light emitting device (LED), would make the base unitmore visible, especially if the visibility is low at the seafloor. Thelight beacon is a simple example of a guidance system associated with ageneric control system.

An autonomous system requires a control system and a guidance systemneither of which include communication to the surface. This implies anautonomous AUV, and eliminates the need for a dedicated human operatorfor steering the AUV from the surface. Of course, a human operator maystill monitor logs, handle software updates etc., but the need for humaninteraction, and hence operational cost, can easily be reduced with anautonomous system.

In many applications, an acoustic positioning system would be suitableas a guidance system. Acoustic positioning systems are available fromseveral vendors, and generally use acoustic sources at fixed locationsfor positioning and orientation. The acoustic sources for thepositioning system can be incorporated in the base units of the presentinvention, or they can be deployed directly at the seafloor. A genericsubsea guidance system could be based on electromagnetic radiation, e.g.visible light or radio. However, the range or visibility can easily beless than the average distance between adjacent base stations.

Alternatively, an autonomous AUV could be provided with a Hall-sensorand a control system capable of following an electric conductor. Such aguidance system can be practical in a sensor field with a pre-existinggrid of steel wires. As noted above, steel wires do not imply powersupply or communication. A possible drawback of such a guidance systemis the need for post-installing a conductive wire, e.g. a thin copperwire, to stand-alone base stations 11 and/or to a sensor field 2 usingstand-alone nodes or synthetic ropes to maintain the distance betweennodes.

The acoustic sources above have different purposes and widely differentproperties, e.g. emitted power and frequency range. However, apart fromthe general term ‘acoustic source’, a seismic source has little incommon with an echo-sounder in a control unit (12) or an acoustic beaconin a base unit (11). The grid of acoustic transducers in an acousticpositioning system is sometimes called an acoustic network. Thus, theterm ‘acoustic network’ does not imply any communication capabilities.However, communication capabilities are not excluded from a genericacoustic network. For example, an acoustic network having its own,separate acoustic transducers may be convenient for communication withinthe sensor field. Similarly, radio links could be employed to provide awireless network at the sensor field 2. Thus, some sensors and/orsources in the sensor field 2 can be connected to a wired network, someto a wireless network and some to no communication network.

To summarise, separating a node into a base unit 11 that is stationaryat the seafloor and a control unit 12 enables subsequent measurements tobe performed at the exact same spot. This increases the accuracy of arepeated measurement, e.g. in time-lapse monitoring. In addition,returning the sensors to their previous location is fast, as finding abase unit 11 is simple compared to finding an exact point usingnavigation equipment. The AUV 10 is an inexpensive means of transport,especially compared to specialised surface vessels. Thus, the system andmethod of the invention improve imaging quality and allow shorterintervals between measurements, and still saves operational costs,especially compared to the operational cost of a surface vessel.

1-10. (canceled)
 11. A system for operating a subsea sensor field formonitoring a region of interest, comprising: an automated underwatervehicle (AUV); a sub sea service station; and a sensor field comprisinga plurality of sensors configured for monitoring a region of interest,wherein at least one sensor in the sensor field comprises a permanentlyinstalled base unit and a removable control unit, the AUV comprises afirst transport connector and the removable control unit comprises asecond transport connector, and the service station comprises a dockingconnector configured for receiving the control unit, and wherein the AUVis configured to autonomously connect and disconnect the first transportconnector to the second transport connector and move the removablecontrol unit between the permanently installed base unit and the dockingconnector on the service station.
 12. The system according to claim 11,wherein the service station is connected to a surface through anumbilical.
 13. The system according to claim 11, further comprising adocking station for the AUV.
 14. The system according to claim 11,wherein the base unit is connected to a wired network.
 15. The systemaccording to claim 14, wherein the wired network is connected to theservice station.
 16. The system according to claim 13, furthercomprising a plurality of base units, wherein the AUV comprises acontrol system and a set of base units comprise a corresponding guidancesystem, such that the AUV is configured to operate without communicationto a surface except through the docking station.
 17. The systemaccording to claim 11, wherein the region of interest is at least one ofa geological structure below the sensor field, a body of water above thesensor field, or the seafloor on which the sensor field is deployed. 18.The system according to claim 11, wherein the control unit comprises atleast one of a battery or storage for data, so that the battery can becharged and/or the control unit undated when the control unit isconnected the docking connector.